Slightly optimize CPU util tracking by defer float conversion

Previously _onSample() computed and stored a volatile float _sma_avg
on every sample (64 times/minute on core 0).  The getter simply returned
that pre-computed value.

This commit inverts the responsibility:

- _sma_sum is now volatile uint16_t — the raw integer sum is the
  cross-core shared state, written atomically by core 0 (single S16I
  instruction on Xtensa LX7) and read by core 1.

- The float conversion (uint16_t → float multiply) is moved into
  getCore0Util(), which is called rarely (once/minute for history,
  on-demand for web requests).  The multiply now happens on the core
  that actually needs the result.

- _sma_avg is removed entirely, saving 4 bytes and one volatile float
  store per sample from the hot path.

- The SMA update is rewritten as a single combined expression:
    const uint16_t last = _sma_buf[_sma_idx];
    _sma_buf[_sma_idx] = s8;
    _sma_sum = (_sma_sum - last) + s8;
  This produces a single write to _sma_sum instead of two (decrement
  then increment), which is cleaner when _sma_sum is volatile.

The cross-core contract is unchanged: core 0 writes _sma_sum once per
sample; core 1 reads it in getCore0Util().  A 16-bit aligned store on
Xtensa LX7 is a single instruction, so no spinlock is needed.

arch/esp32/CPUUsageTracker.cpp

@@ -30,11 +30,10 @@ void CPUUsageTracker::_onSample() {
   const float sample = (total > 0) ? (float)db / (float)total : 0.0f;
 
   const uint8_t s8 = (uint8_t)(sample * 255.0f + 0.5f);
-  _sma_sum -= _sma_buf[_sma_idx];
+  const uint16_t last = _sma_buf[_sma_idx];
   _sma_buf[_sma_idx] = s8;
-  _sma_sum += s8;
+  _sma_sum = (_sma_sum - last) + s8;
   _sma_idx = (_sma_idx + 1) & (SMA_WINDOW - 1);
-  _sma_avg = (float)_sma_sum * (1.0f / (SMA_WINDOW * 255.0f));
 }
 
 void CPUUsageTracker::begin() {